Best way to wire a split charge relay?

Been wrestling with this for my shepherd's hut build. Currently running a 200W solar array charging a leisure battery via a basic PWM controller, but I'm adding a second battery for the cabin and want to get the split charge sorted properly.

I've got two 100Ah LiFePO4s I want to wire up so the solar tops both off, but they stay isolated when the engine's running or when one's depleted. Reading conflates me a bit though — some folks swear by a traditional relay, others are saying VSR (voltage sensitive relay) is the way, and I've seen mentions of Victron's smart solutions that seem overkill for my setup.

My questions:

1. For dual leisure batteries fed by solar only (no vehicle alternator), is a basic split charge relay actually necessary? Could I just wire them in parallel to the controller output?

2. If I do need a relay, what amperage rating should I be looking at? My array is 200W which maxes out around 10-12A in ideal conditions, but I'm planning to expand.

3. Anyone using a Fogstar or similar UK-sourced relay they'd recommend?

I've seen the Victron Cyrix stuff mentioned but that feels pricey for what I need. Equally, I don't want to bodge it and end up with a dead battery mid-winter.

Cheers for any pointers. This is the last piece before I move the setup into the hut full-time.

Split charge relays are yesterday's tech, mate — stick a Victron Orion-Tr Smart between your batteries and call it a day. Costs more upfront but you'll actually know what's happening via the app instead of wondering if your relay's stuck again at 2am.

If you're dead set on the relay route, wire it so the leisure battery charges first (your priority), then the cabin battery tags along once you hit ~13.2V. Prevents your van going dark when you're miles away.

That said, with 200W of panels you're already stretching things thin for two batteries. What's your actual usage looking like?

Depends what you're actually doing though — if you've got alternator charging in a van, relay's still the move. For your shepherd's hut setup though, @BatteryAlan's got a point about going DC-DC if you're solar-only, keeps everything isolated and stops backfeed nonsense. That said, a Redarc SBI12 or basic VSR does the job for pennies if your leisure battery's already in good nick. Just don't cheap out on the cable — 4mm² minimum run from battery to battery or you'll spend three years wondering why your second leisure pack never charges properly. What's your actual charging source, alternator or solar?

Mate, if you're running solar only like you say, you don't even need a relay — just parallel the batteries with a decent diode isolator or go full Victron Orion-Tr like @BatteryAlan suggests if you want smart charging prioritisation. The relay only makes sense if you've got engine charging throwing lumpy voltage at things.

That said, if you're dead set on a relay for peace of mind, a classic VSR from Redarc does the job without overthinking it. Just make sure your second battery's got its own fuse — learned that one the hard way when my shepherd's hut nearly became a shepherd's crisp.

What's powering your second battery — more panels or are you thinking alternator down the line?

The issue with just paralleling them like @NotAnElectrician80 suggests is you'll end up with voltage sag across both banks during discharge, especially if they're different ages or capacities. That said, @BatteryAlan's got a point about relays being dated if you're purely solar.

For a shepherd's hut though, I'd honestly go with a simple VSR (voltage sensitive relay) rather than the Orion if budget's tight — your 200W array isn't going to justify the smart unit. Something like a Redarc SBI12 or even a basic split charge relay from Fogstar will handle solar top-ups without faffing about.

The relay drops out when you're drawing from the leisure battery, so your cabin battery stays isolated and doesn't get drained when someone's running the kettle. Clean setup, cheap, and does exactly what you need.

Wire it so the leisure battery feeds one side, cabin battery the other, relay triggers on alternator or solar charge voltage. Job done.

@WingAndPrayer the relay debate here depends entirely on your charging source. Since you're solar-only with PWM, you're actually in the simpler camp — @NotAnElectrician80's right that you don't strictly need a relay, but the reasoning matters.

The issue @PennineNomad raises about voltage sag is valid if you're drawing heavily from both banks simultaneously. With a 200W array that's probably not your immediate concern, but it's worth planning ahead.

What I'd actually suggest: wire both batteries to the same charge controller output using properly sized cable. The PWM will regulate voltage to both regardless. This keeps things straightforward and cheap. Only consider a split charge relay later if you add alternator charging to a vehicle, which changes the whole picture.

What's your load profile looking like on the cabin side? Are you planning to draw simultaneously from both banks, or is one primarily for backup? That'll determine whether you need the complexity of a relay setup or if you're overthinking it like most people do on these builds.

Been down this road with my static caravan setup. The real question isn't whether you need a relay—it's what happens when you don't have one and your cabin battery starts discharging back through the leisure battery at night.

@NotAnElectrician80's got a point about simplicity, but @PennineNomad's voltage sag concern is spot on with a 200W array. What I'd actually suggest is looking at a VSR (voltage sensitive relay) rather than a traditional split charge. With my Victron setup, the VSR only connects the batteries when the solar's actively pushing voltage above a threshold—so no parasitic drain, no relay clicking away at night.

If you're set on a basic relay, make sure it's rated for your expected charge current. A cheap automotive unit will struggle once you upgrade that solar array (and you will).

Alternatively, keep them completely separate with individual PWM controllers if your wiring can handle it—more batteries, more flexibility down the line. That's what I'm planning for the cabin expansion anyway.

What's your cable gauge running currently?

Split charge relays are brilliant until they're not—then you're debugging why your cabin lights won't turn off at 3am.

Since you're PWM solar-only, honestly? A Victron Orion-Tr isolated DC/DC charger would solve this more elegantly than relay gymnastics. Keeps batteries isolated, handles your voltage sag issue that @PennineNomad mentioned, and you get proper multi-stage charging on both banks.

If you're set on a relay, voltage-sensitive types (like a Redarc SBI12) are idiot-proof compared to split-charge relays—just watch your wiring gauge doesn't let voltage drop fool the sensor.

The shepherd's hut advantage is you've actually got space to run proper cable runs without the "oh god, where does this go" panic of a van conversion. Don't cheap out on the 2/0 AWG equivalent just because you can fit it.

@WingAndPrayer—the catch with PWM + split charge relay is that your controller won't know the leisure battery's state once it's isolated. You'll end up with an undercharged cabin battery whilst the main one sits at 13.2V.

Worth considering a VSR (voltage sensitive relay) instead—something like the Redarc SBI12 or a basic Narva unit. They key on the voltage rise when charging starts, so both batteries charge together, then isolate when voltage drops. Dead simple, no complexity.

That said, if you're planning to upgrade to MPPT down the line (which you should, with 200W), you could just run both batteries in parallel permanently with adequate cable gauge. Depends if you need independent loads.

What's your current cable run between the two batteries? That'll determine whether you can get away with just better wiring, or if you genuinely need the isolation for voltage drop reasons.

The PWM controller issue @WattEd mentioned is the real gotcha here. Your leisure battery will charge fine, but the relay triggers based on voltage alone—so if your cabin battery's sitting at 13.2V whilst the leisure one's still climbing, the relay might not engage when you'd expect it to.

Worth considering: what's your actual use case? If the cabin battery's just for lights and you're not running high loads, a simple VSR (voltage sensitive relay) will do the job. But if you're planning proper discharge cycling on both banks, you'd be better served by a split charge diode or upgrading to an MPPT controller that can handle multiple battery monitors.

I went diode route on my van conversion initially—dead simple, no moving parts to fail at 3am like @FETFan says. Only downside is the voltage drop (0.7V), but with a 200W array you're not starved for charge anyway.

What's the leisure battery spec, and how much current are you planning to pull from the cabin setup?

The PWM thing's a proper headache, yeah. What you actually want is a VSR (voltage sensing relay) instead—Victron make a solid one. It'll monitor both batteries and only connect them when the primary's properly charged, then disconnect before your leisure battery drains the cabin one flat.

With your 200W array though, I'd honestly consider skipping the relay altogether and going MPPT with dual outputs if budget stretches that far. Renogy do a decent one that handles priority charging. Then both batteries get proper management and you're not relying on voltage thresholds at 3am like @FETFan said.

If you're wedded to the split relay, at minimum wire it so the leisure battery can only charge from the vehicle circuit, never the other way round. Saves the drama of one battery killing the other.

What's the second battery spec? If it's similar capacity to your leisure one, a VSR makes more sense than PWM juggling.

VSR's the way to go tbh. Been running one on my motorhome setup for ages—dead reliable. The Victron Cyrix-ct is pricey but worth every penny if you've got the budget, otherwise the Redarc SBI12 does the job for half the cost.

Main thing is the VSR watches your main battery voltage and only connects the second battery when charge is decent (usually around 13.2V), then disconnects when you're drawing down. Means your PWM controller can keep doing its thing without confusion.

Only real consideration: make sure your wiring's chunky enough for both batteries. I initially underestimated the cable gauge on mine and got voltage drop gremlins. Go thicker than you think you need.

What size leisure battery you adding? If it's similar capacity to your main one, the VSR will balance the charge pretty well on its own.

VSR's definitely the move. I've got a Fogstar one on my setup and it's solid—way less faffing about than manual relays. Only thing, make sure your second battery's similar capacity to the first or you'll get odd charging behaviour. What size is the cabin battery you're looking at?

VSR's definitely the answer here. I've got a Victron Cyrix-li on my garden office setup—works a treat. Just wire it between your batteries and let it handle the switching automatically. Dead simple, no manual intervention needed. Only thing: make sure your cables are properly sized for the load, otherwise you'll get voltage drop issues.